This invention relates generally to fault detection, in load bearing structures, and more particularly concerns improvements in methods and apparatus for determining locations of damage or faults in such structure, one example being bridges.
1. Introduction
To safeguard the safety performance of a civil infrastructure, such as a bridge, regular inspections are essential. At the present time, the inspection method is primarily visual. An experienced engineer or technician has to go through a bridge to examine each member and certify its safety. This method is subjective and flawed, for lack of rigorous standards. For example, for a bridge deteriorating from fatigue or aging, the damage is usually not clear-cut. Therefore, any call is judgmental. Furthermore, it is not feasible to use this visual method for complicated bridge structures; there might be members located at positions too awkward to access; there might be too many members that would require too much time to inspect; and there might be damage too subtle to detect visually. Because of these limitations, the visual inspection results are known to be incompletely reliable; yet, inspectors are forced to rely on it, today.
The safety of the bridges, however, is too critical and urgent a problem to be left in the present state for long; and it is a crisis of gigantic proportions considering the aging of many thousands of bridges. Since the greatest highway construction period of the US was in the late nineteen fifties and in the nineteen sixties, many bridges are reaching their service limitation at this time. In a recent survey by the Federal Highway Administration (Chase and Washer, 1997), 37% of all bridges in service were found to have some degree of structural deficiency. That percentage is increasing fast with the aging of the bridges; therefore, a reliable strategy has to be devised so that limited financial resources can be effectively employed in response to this national infrastructure crisis.
Ideally, any inspection method will have to satisfy the following conditions:
1. To be robust, objective, and reliable. PA1 2. To be able to identify the existence of damage. PA1 3. To be able to locate the damage. PA1 4. To be able to determine the degree of the damage. PA1 a) providing a transient load imposing device, PA1 b) traveling that device onto the bridge and then along the bridge, PA1 c) detecting bridge vibrating response including changes in frequency during such traveling, PA1 d) and determining the position of the device in relation to a shift of frequency below a selected value.
Visual inspection methods have clearly failed the first requirement, and thus have introduced uncertainties in the rest of the requirements. This requires the conclusion that non-destructive inspection methods should employ precise scientific sensors coupled with rigorous data analysis. That approach has been the central theme of research in the Bridge Management Program, Turner-Fairbank Highway Research Center, Federal Highway Administration. A large research program of research and development in new technologies for the nondestructive evaluation of highway bridges has been initiated. The objectives are locating, quantifying, and assessing the degree of damage of the bridges in supporting of the bridges. Although various technologies have been developed, such as Infrared Thermography, Ground-Penetrating Radar, Acoustic Emission Monitoring, Eddy Current Detection and others, none of them are practical. The difficulties of such systems are due, mostly, to their limited field of view. One must locate the damage first before he can use sophisticated imaging devices to examine the damage, in detail. For a complicated structure, locating the damage is more than 90% of the job. As a result, even with the advances of these esoteric techniques, the data used in bridge management today is still based almost entirely upon unreliable visual inspection.